The present invention relates to a process for making a uniform flat, and especially a large, press plate, particularly for a screen printing platen.
In a screen printing machine with a stationary press member (screen) and a movable wiper element, a stationary press platen is provided for support of the materials to be printed, e.g. stuff, glass, enamel surfaces and the like. This press platen is, for example, movable for proofing. It has on its upper side a press contacting or pressing surface, which must be flat to guarantee a high quality screen printing. In the known screen printing machine, the press plate of the press platen is made of metal, particularly from eloxated aluminum. In making of a vacuum press platen, the press plate is formed as a vacuum press plate, which contains a plurality of throughgoing holes or perforations. Also in case of the vacuum press plate the press plate can be made of metal, particularly eloxated aluminum. This type of press plate is above all heavy in large sized machines, expensive and delicate regarding its flat surfaces. In operation, damage of the press surfaces can not be prevented. Then it is necessary to repair the press surface of the press plate, i.e. to make it uniformly flat and to eloxate it as needed. This is expensive and requires much effort. Moreover, this procedure takes a certain time interval, in which the screen printing machine cannot be used, so that, above all, this causes a certain idle or down time, which is considerably disadvantageous.
Press plates are known which have thin formica layers glued to their surfaces. These formica layers are hard, sensitive and brittle so that they are very easily damaged. Repair of these surfaces is not possible, because in a very time consuming and expensive process the formica layers which are damaged must be first removed from the support plate, the remaining formica regions on the press plate being easily broken and very difficult to remove. In this case the new press plate must also be mounted in the press which is also expensive and difficult. Here also there is no guarantee that the surface of the press plate maintains its high quality. For both types of the known press plate, the danger exists that scratch-sensitive workpieces to be printed, e.g. glass plates, enamel surfaces, and the like, can be comparatively easily scratched. Also the surface of this kind of press plate is, for its part, also very scratch sensitive.
A screen printing platen with a press plate mounted on a frame for holding a workpiece to be printed is known from German Published Patent Application 38 23 853, in which the surface of the press plate is covered by an adhesive plastic foil. Because of this plastic foil, damage to the press plate can be avoided. The plastic foil should, be easily replaceable, when it is damaged. Also the replacement of the plastic foil is inexpensive and does not take much time. However the foil fits and follows the surface of the support plate, so that no flat surface is formed, as is required by the printing process, if there were nonflat regions on the press plate. Also the danger exists that the plastic foil is easily and quickly loosened from the press plate, especially under the action of temperature, solvent and/or mechanical forces.